The widespread deterioration of road structures, specifically bridges, has been acknowledged as a critical problem in our Nation's transportation system. The Federal Government considers hundreds of thousands of bridges structurally deficient or functionally obsolete. A major factor in the problems of bridges are bridge decks, whose life span averages only one half the service life of the average bridge.
The rehabilitation and redecking of existing deficient structures, as well as deck designs for new structures, must account for many factors affecting bridge construction and rehabilitation. These factors include increased usage, increased loading, reduced maintenance, increased use of salts for snow, and the need for lower costs, lighter weight, and more efficient construction techniques. Prior to the advent of exodermic decks, the available deck designs included some specific beneficial characteristics, but none have all of the features required to meet current needs. U.S. Pat. Nos. 4,531,857, 4,531,859, 4,780,021, and 4,865,486 disclose exodermic decks and exodermic deck conversion methods which have met all the above design factors with unparalleled success.
An exodermic or "unfilled, composite, steel grid" deck consists of a composite concrete component and a steel grid component. A thin, reinforced concrete component is cast above an open, unfilled grid component forming a composite deck section. Shear transfer elements from the grid component are embedded into the concrete component providing the capability to transfer horizontal shear forces between the reinforced concrete component and the steel grid component and preventing vertical separation between the concrete component and the steel grid component.
An exodermic deck achieves enhanced composite behavior. Also, in a typical exodermic construction, the neutral axis of the composite deck is relocated near the top of the grid component. This reduces the maximum stress level in the top surface of the grid component to a point at which fatigue failure should not occur. An exodermic deck maximizes the use of the compressive strength of concrete and the tensile strength of steel to significantly increase the deck section properties over that of known conventional deck constructions of equal weight. The advantages achieved by exodermic decks also include reduced weight, rapid installation, increased strength, longer expected life and increased design flexibility.
Exodermic decks can be lighter than conventional decks of comparable load design. This reduction of weight results in significant savings on new steel framing and substructures and significantly upgrades the live load capacity of existing bridges. A further benefit achieved by the reduction of weight is the favorable effect on the fatigue life of bridge members.
Structural testing to date has shown that exodermic decks can be expected to have a fatigue life in excess of other deck configurations at comparable load design capacities. An exodermic deck eliminates potential fatigue failure thereby extending the useful life of the deck.
Additionally, exodermic bridge decks can easily be designed for numerous varying size and strength requirements. Exodermic decks can be cast-in-place or prefabricated in sections and transported to the site for installation. A cast-in-place exodermic deck provides a continuous concrete surface which can be maintained in the same manner as any reinforced concrete deck, at significantly lower weight. Exodermic decks which are prefabricated in sections permit rapid installation without regard to the weather and create the ability to utilize an off-site rigid quality control system for the deck.
Moreover, an exodermic deck eliminates skidding and noise problems commonly associated with open grid deck bridges and with filled grid deck bridges which do not have a wearing surface above the grid.
An exodermic deck design, used on all installations to date, includes a concrete component and a steel grid component comprised of main bearing bars, secondary or distribution bars, and tertiary bars. Short vertical dowels or studs are preferably welded to the tertiary bars. The top portion of the tertiary bars and the vertical dowels welded thereto are embedded in the concrete component to transfer the shear forces between the concrete component and the steel grid component and prevent any vertical separation between the concrete component and the steel grid component.